I dont want to be a brainlet, I want to live a meaningful existence but I feel as if im too old to make any significant change in terms of my intelligence. Is it true that at 25 the brain stops growing and from that point its just cognitive decline?

All geniuses seemed to show themselves at young ages doing extraordinary feats, even standard scientists seem to of all been recognised for their intelligence at young ages.

Am I fucked basically? Im eating clean, exercising, trying to fight being a general depressed loser but theres this nagging sense that time has won.